There is shown in U.S. Pat. No. 3,639,010, issued Feb. 1, 1972 to Gary T. Mayer et al, and assigned to the assignee of the present application, a graduated release type brake control valve device that is operable in response to a reduction of the brake pipe pressure effective on one side of an abutment relative to a substantially constant pressure in a control reservoir that is charged from the brake pipe via a one-way flow valve, there being no means disclosed to provide for flow of fluid under pressure from the control reservoir to the brake pipe should the pressure in the control reservoir exceed the pressure in the brake pipe.
As a practical matter, the volume of the train brake pipe is considerably in excess of the volume of the control reservoir on each individual car in a train. Moreover, the pressure in a normally charged train brake pipe is maintained at a substantially constant value by a self-lapping type control valve device that is embodied in the engineer's brake valve device located on a locomotive for controlling operation of a brake control valve device provided on each car in the train hauled thereby. Therefore, upon an increase in the temperature of the ambient air, the resulting increase in the pressure of the fluid in each control reservoir will be considerably in excess of the increase of the pressure in the train brake pipe. Accordingly, this excess of control reservoir pressure over that of train brake pipe pressure will cause the brake control valve device on the cars to operate to effect an undesired brake application.
Moreover, the normal fully charged pressure carried in the brake pipe and, therefore, in the control reservoirs of the cars in one type of train, such as a passenger train, is considerably in excess of the normal fully charged pressure carried in the brake pipe of another type of train, such as a freight train.
Furthermore, the normal fully charged pressure carried in the brake pipe of any particular type of train operated by one railway company may be considerably in excess of the normal fully charged pressure carried in the brake pipe of the same type of train operated by another railway company. Accordingly, a brake release is not obtained when a railway car set off from a train having a high normal fully charged brake pipe pressure is subsequently coupled into a train having a lesser normal fully charged brake pipe pressure for the reason that this pressure is less than the pressure of the fluid in the control reservoir on this car.
Also, as a practical matter, occasional erratic operation of the self-lapping type control valve device that is embodied in the engineer's brake valve device will cause the pressure maintained in the brake pipe to be less than the normal fully charged pressure carried therein. Therefore, assuming that the control reservoir has been charged to the normal fully charged pressure carried in the brake pipe prior to erratic operation of the self-lapping control valve device to maintain a lesser pressure in the brake pipe, it is apparent that this lesser pressure maintained in the brake pipe is ineffective to cause a complete brake release which, of course, is undesirable.
Accordingly, it is the general purpose of the present invention to provide an abutment that operates a brake control valve device in response to a reduction in brake pipe pressure on one side thereof relative to control reservoir pressure on the opposite side with a valve mechanism and a choke that are operatively effective in response to an increase in the pressure of the fluid under pressure in the train brake pipe to a value in excess of the pressure of the fluid under pressure present in the control reservoir to establish a communication via which fluid under pressure may flow from the brake pipe to the control reservoir to effect charging thereof, and operatively effective to close this communication upon a subsequent decrease in the pressure of the fluid under pressure in the brake pipe thereby rendering the brake control valve device operative by the pressure in the control reservoir to effect a brake application.